Logging method and system for measuring mechanical parameters of the formations crossed through by a borehole

Abstract

The present invention relates to a method for obtaining a recording or log of measurements substantially comparable to the acoustic impedance of the formations crossed through by the borehole. The measurements are obtained from an acceleration detector (11) placed in the drill string (2) between the rock-breaking bit (7) and mechanical uncoupling system (12) set between certain elements of the string. A measurement representative of the acoustic impedance of the drilled formations is obtained by processing the signal by a function of the rotating speed of the drill bit and of the depth.

We claim: 
     
       1. A method of geological exploration of formation comprising drilling a borehole to a depth into the formation, said drilling being conducted by a drill bit (7) fastened to the end of a drill string (2), said string comprising measuring means (11) delivering a signal representative of an acceleration created by said bit, wherein mechanical uncoupling means (12) are placed in the drill string, said measuring means are set between said drill bit (7) and said uncoupling means (12), and a measurement representative of the acoustic impedance of the drilled formation is obtained by processing said signal by a function of the rotating speed of the drill bit and of the depth of the borehole. 
     
     
       2. A method as claimed in claim 1, wherein the acceleration measurement is considered in a time window ranging between 5 and 30 s, said processing comprising: calculating in this window an effective amplitude representative of said acceleration signal,   dividing said amplitude by the rotating speed of the drill bit, and optionally by the rate of penetration raised to a power of about 0.1,   and multiplying said amplitude by aPf+b, where a and b are constants related to the borehole environment, notably the depth, and Pf the bottomhole pressure.   
     
     
       3. A method as claimed in claim 2, wherein the acceleration measuring means (11) comprises at least one accelerometer placed so as to measure a signal representative of the axial acceleration substantially, along the axis of the borehole. 
     
     
       4. A method as claimed in claim 2, wherein pore pressure changes in the drilled formations are deduced from variations in said measurement representative of the acoustic impedance. 
     
     
       5. A method as claimed in claim 4, wherein seismic pickups from which seismic uphole times are deduced are placed at the surface, the source being the noise of the drill bit, characterized in that a synthetic seismic response in uphole two-way time is calculated from said measurement representative of the acoustic impedance and said uphole times. 
     
     
       6. A method of claim 2, wherein seismic pickups from which seismic uphole times are deduced are placed at the surface, the source being the noise of the drill bit, characterized in that a synthetic seismic response in uphole two-way time is calculated from said measurement representative of the acoustic impedance and said uphole times. 
     
     
       7. A method as claimed in claim 2, wherein acceleration measuring means comprises at least one accelerometer placed so as to measure a signal representative of the rotation acceleration. 
     
     
       8. A method as claimed in claim 1, wherein the acceleration measuring means (11) comprises at least one accelerometer placed so as to measure a signal representative of the axial acceleration, substantially along the axis of the borehole. 
     
     
       9. A method of claim 8, wherein seismic pickups from which seismic uphole times are deduced are placed at the surface, the source being the noise of the drill bit, characterized in that a synthetic seismic response in uphole two-way time is calculated from said measurement representative of the acoustic impedance and said uphole times. 
     
     
       10. A method as claimed in claim 8, wherein acceleration measuring means comprises at least one accelerometer placed so far as to measure a signal representative of the rotation acceleration. 
     
     
       11. A method as claimed in claim 8, wherein pore pressure changes in the drilled formations are deduced from variations in said measurement representative of the acoustic impedance. 
     
     
       12. A method as claimed in claim 1, wherein the acceleration measuring means comprise at least one accelerometer placed so as to measure a signal representative of the rotational acceleration. 
     
     
       13. A method as claimed in claim 1, wherein changes in the nature of the drilled formations are determined from variations in said measurement representative of the acoustic impedance. 
     
     
       14. A method as claimed in claim 1, wherein pore pressure changes in the drilled formations are deduced from variations in said measurement representative of the acoustic impedance. 
     
     
       15. A method of claim 14, wherein seismic pickups from which seismic uphole times are deduced are placed at the surface, the source being the noise of the drill bit, characterized in that a synthetic seismic response in uphole two-way time is calculated from said measurement representative of the acoustic impedance and said uphole times. 
     
     
       16. A method as claimed in claim 1, wherein seismic pickups from which seismic uphole times are deduced are placed at the surface, the source being the noise of the drill bit, characterized in that a synthetic seismic response in uphole two-way time is calculated from said measurement representative of the acoustic impedance and said uphole times. 
     
     
       17. A method as claimed in claim 1, wherein the acceleration measurement is considered in a time window ranging between 5 and 30 s, said processing comprising: calculating in this window an effective amplitude representative of said acceleration signal,   dividing said amplitude by the rotating speed of the drill bit and by the rate of penetration raised to a power of about to 0.1,   and multiplying said amplitude by aPf+b, where a and b are constants related to the borehole environment, notably the depth, and Pf the bottomhole pressure.   
     
     
       18. A method as claimed in claim 17, wherein the acceleration measuring means (11) comprises at least one accelerometer placed so as to measure a signal representative of the axial acceleration, substantially, along the axis of the borehole. 
     
     
       19. A method as claimed in claim 17, wherein pore pressure changes in the drilled formations are deduced from variations in said measurement representative of the acoustic impedance. 
     
     
       20. A method as claimed in claim 17, wherein seismic pickups from which seismic uphole times are deduced are placed at the surface, the source being the noise of the drill bit, characterized in that a synthetic seismic response in uphole two-way time is calculated from said measurement representative of the acoustic impedance and said uphole times. 
     
     
       21. A method as claimed in claim 17, wherein acceleration measuring means comprises at least one accelerometer placed so as to measure a signal representative of the rotation acceleration.